Twenty neptunium radioisotopes have been characterized, with the most stable being 237Np with a half-life of 2.14 million years, 236Np with a half-life of 154,000 years, and 235Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has 4 meta states, with the most stable being 236mNp (t1/2 22.5 hours).

Electron capture: the decay energy is 0.125 MeV and the decay product is Uranium-235

This particular isotope of neptunium has a weight of 235.0440633 u.

Neptunium-236

Neptunium-236 has 143 neutrons and a half-life of 154,000 years. It can decay by the following methods:

Electron capture: the decay energy is 0.93 MeV and the decay product is Uranium-236.

Beta emission: the decay energy is 0.48 MeV and the decay product is Plutonium-236.

Alpha emission: the decay energy is 5.007 MeV and the decay product is Protactinium-232

This particular isotope of neptunium has a mass of 236.04657 u. It is a fissile material with a critical mass of 6.79 kg.[6]

236Np is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237Np,[7] however it is nearly impossible to separate in any significant quantities from its parent 237Np.[8] It is for this reason that, despite its low critical mass and high neutron cross section, it has not been researched as a nuclear fuel in weapons or reactors.

Neptunium-237

In 2002, 237Np was shown to be capable of sustaining a chain reaction with fast neutrons, as in a nuclear weapon, with a critical mass of around 60 kg.[9] However, it has a low probability of fission on bombardment with thermal neutrons, which makes it unsuitable as a fuel for conventional nuclear power plants (as opposed to accelerator-driven systems, etc.).

Use in plutonium-238 production

When exposed to neutron bombardment 237Np can capture a neutron and become 238Pu, this product being useful as an thermal energy source for the production of electricity in deep space probes and, of recent note, the Mars Science Laboratory (Curiosity rover). These applications are economically practical where photovoltaic power sources are weak or inconsistent.

Notes

Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.

Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.

References

↑Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no isotopes have half-lives of at least four years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.

↑Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. doi:10.1016/0029-5582(65)90719-4.
"The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 y. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."

↑This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".

↑Excluding those "classically stable" isotopes with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.